I'm leaving in a month, so why should I care much?
I don't care much, so I'm leaving at the end of the month.
It's hard to get into a habit of virtual materialism when it's all going to be gone soon. What is an epic when you're offline? Nothing at all. Nothing beside the stories and social ties exist outside the game. Depending on the individual, neither of those may either.
If you were going to be quitting in a month, would you worry much about that last rep grind or rare drop? Probably not. Under certain circumstances you could have a surge of activity, seeking to complete a challenge before time runs out, such as a hardmode, but if you don't realistically think your guild can complete it before then, then the opposite may occur. Even worse, if they think you'll be gone soon, they may catch the Curse of the Temporary as well.
In a Catch-22 situation, it is precisely these long-term goals, this investment in a game beyond a few hours of immediate fun, which would give reason to subscribe for another month. Without them, you won't. If long-term goals appear unachievable, or without a dramatic change, such as a reroll or dropping a long-time guild, then even if a player cares, they may not care enough.
Now imagine if this happened to more than one person ever. Imagine if the Curse of the Temporary spreads, and stays. With ever-smaller raids, missing one player is an ever-bigger setback. Lose too many and long-term goals may appear unachievable. That drives away more players. They go elsewhere. When they get there, they bring memory, of the temporary nature of all things MMO, and so again they seek the one month of a bit of fun and when it runs out they leave.
A vast migration develops as players move from game to game, never committing to any. Those who remain anywhere find themselves in a game with hundreds of thousands, even millions of players, but constantly churning, and never sure what they can accomplish when players vanish and reappear a few months later. Developers follow suit, drumming up new short-term goals to grab the next migration from the last game that ran out of short-term goals. Meanwhile, anyone seeking to play for months finds himself wondering where everyone went and how he is supposed to do anything at all when everything can collapse with a single patch in another game.
He's not doomed. There are other stable players, but fewer and fewer all the time. It's not that the game he plays is dying (though it could be too), but that those who play it play it differently. So the game in the game he plays is dying, and as it dies, more leave and fewer come.
And then he says, "screw it" and sees the date when his game time will run out, an unknown expanse of nothingness beyond.
His friends call to him from other games, a dozen other titles with stories and class all of their own, or no stories and no classes. He could go off, learn their ways, play a month or two, and then forget it all to rush off to a new reality with new rules. An endless chase around the worlds, each one more fun than the one before, and dying faster than the one before, until he wonders where he got at all. How much happier he was in his illusion of one permanent world, changing, but not vanishing. He knew it too would die, someday, but not so soon, and so he could believe and fool himself and enjoy the illusion that it would last, because strangely, as long as everyone believed it, it was true. Until they didn't.